I had some difficulties installing the guest addition in my centos 6.5 VM so after upgrading to VirtualBox 4.3.8 I searched a bit and found an article that pointed me in the right direction.
Everyone points you to click on the Devices menu and then on Insert Guest Addition CD…
I then needed to:
sudo mount /dev/cdrom /media
and I could then
Since I already had the dkms module installed everything worked as expected.